1. Field of the Invention
The present invention relates to a flexible printed circuit board having a substrate formed of plastic film or the like and coated with a conductive wiring layer. The present invention also relates to a substrate coated with a conductive layer, such as a transparent film having a transparent conductive layer or the like, and a manufacturing method thereof.
2. Description of the Related Art
Along with the recent increasing trend toward smaller, lighter, and thinner electronic equipment, there is more need for multipurpose flexible printed circuit boards made very thin and pliable. Conventional flexible printed circuit boards of this type have generally been constructed using polyester film or polyimide film as the substrate and copper leaf affixed to the surface of the substrate by adhesive.
Along with the increasingly high integration of semiconductor devices, however, there has been a strong demand for a low-cost flexible printed circuit board with closely packed interconnects between electronic devices incorporated in semiconductor devices. Therefore, studies have been made on methods for manufacturing reliable printed circuit boards to replace conventional techniques that use adhesives. One of these techniques employs ion plating or sputtering to deposit copper film directly on the surface of the substrate, such as plastic film. With this technique we can manufacture a flexible printed circuit board having strong adhesion without requiring adhesive. When manufacturing a printed circuit board using the cluster ion beam method to deposit copper onto the surface of polyimide film, for example, the technique achieves a precise pattern and great durability with sufficient adhesion strength and flexibility. The resulting product has a performance far superior to conventional printed circuit boards manufactured with adhesives.
However, a high volume production of flexible printed circuit boards by depositing thin copper film on the surface of a high polymer substrate using the ion beam method would require massive vacuum equipments. Such equipment would normally necessitate a high initial investment.
Transparent conductive film formed over transparent plastic film is widely used as an electrode layer in solar cells, display devices, photoelectric transducers, and the like. This conductive film has such electrical applications as transparent planar heating elements and anti electrostatic displays, and such optical applications as heat-rays shielding windows and selectively transmissive membranes for solar collectors and has functions that can be applied to a broad range of fields. Here, transparent conductive film is defined as the film having both transparency and conductivity formed on the substrate of transparent high polymer resin, which is in essence an electrical insulator.
The above-mentioned transparent conductive film requires not only a combination of high conductivity with good transparency for visible rays, but also high reflectivity for infrared ways. Conductive film can be broadly classified as practical metal films and oxide films. Examples of the former include Au, Ag, Cu, Pd, Pt, Al, Cr, and Rh films, while those of the latter include In2O3, SnO2, Cd2SnO4, and CdO films. Among these, silver (Ag) is enjoying increasing use as a selective conductive layer for electrode and the like to make use of its high electric conductivity.
When a thin gold film is used as a transparent conductive film, the sheet resistance of the conductive layer is 10-102 xcexa9/xe2x96xa1, with a high transparency of 70-80%. If, however, a polyester film were to be used as a substrate, adhesion would not be favorable. Therefore, a thin Pd film is used even though conductivity and transparency are slightly lower than those of Au film. In this case, when employing a transparent conductive film that uses thin metal film, the general practice is to form the thin metal film using sputtering deposition. As an examples of a transparent conductive film that uses an oxide semiconductor film, the sheet resistance of tin-doped indium oxide (indium tin oxide: ITO) film is approximately 10-105 xcexa9/xe2x96xa1, and the transparency, 80-88%. Although, the chemical stability of the material is slightly inferior to a SnO2 film, this material is widely used, since the transparency and conductivity are both superior.
The vacuum deposition technique using indium oxide with a tin additive and the sputtering technique are employed in the formation of ITO film. Another film consists of a sandwich structure of a thin metal film interposed between oxide semiconductor films. This film, TiO2/Ag/TiO2, is known to have an excellent conductivity (sheet resistance of 1-10 xcexa9/xe2x96xa1) and a high transparency of 75-85%. Namely, conductivity and transparency are both superb. In addition, high polymer electrolyte film has a low conductivity (sheet resistance of 106-1010 xcexa9/xe2x96xa1, dependent on humidity) and a transparency of 80-85%.
This kind of transparent conductive film is produced by depositing thin transparent and conductive films on a high polymer base film serving as a substrate. This kind of conductive film is produced mainly using such conventional dry deposition techniques as vacuum deposition and reactive sputtering.
When using these conventional methods for covering plastic film with a layer of silver or other thin metal film to form transparent conductive film, it is necessary to keep the deposition temperature low mainly due to the limited heat resistance of the plastic film. Accordingly, both materials may have poor adhesion. A difference in the coefficient of thermal expansion of the two materials may develop thermal stress and distortion, which create adhesion problems. To avoid these problems, undercoat depositions and overcoat deposition are being regularly done as pre-treatments and post-treatments respectively in addition to the main deposition process. Under these conditions, however, the production process becomes more complicated. Moreover, forming films through vacuum deposition or sputtering inevitably requires special large vacuum equipments, which give rise to the problem of higher equipment costs.
In view of the foregoing, it is an object of the present invention to provide a flexible printed circuit board or a transparent substrate coated with a transparent conductive layer having high reliability and improved adhesion. It is another object of the present invention to provide a method for manufacturing the substrate coated with the conductive layer that does not employ adhesives or large, expensive vacuum equipments.
According to an aspect of the present invention, there is provided a substrate coated with a conductive layer, which comprises a substrate having a top surface; and a conductive layer of bonded ultrafine metal particles formed on the top surface of the substrate. It is desirable that the ultrafine metal particles have a diameter of 1-20 nm, and that the substrate is of a flexible high polymer material.
Since the conductive layer is formed by bonded layer of the ultrafine metal particles, an extremely thin layer having high conductivity can be formed. This structure enables the formation of a substrate coated with a conductive layer, such as a flexible printed circuit board with densely packed interconnects or a transparent conductive film provided with both transparency and conductivity. By selecting an appropriate combination of the ultrafine particle layer and the film for the substrate, the two materials are capable of mixing together along their bonding interface. In this structure, a portion of the ultrafine particles of silver or the like become embedded in the surface layer of the base film, forming strong adhesion between the metal layer and the base film.
According to another aspect of the present invention, a method for manufacturing the substrate coated with a conductive layer, comprises: preparing a substrate; preparing an ultrafine particle solution having ultrafine metal particles dispersed in a prescribed solvent; introducing the ultrafine particle solution onto a surface of the substrate; and bonding the ultrafine metal particles by drying the ultrafine particle solution introduced onto the surface of the substrate through subsequent heating process.
Since a strong bond is generated between the ultrafine metal particles and between the base film and the ultrafine metal particles at a relatively low temperature of 200-300xc2x0 C. or less, a conductive layer having high conductivity and high adhesion can be formed on the plastic film or the like without generating problems with thermal stress and distortion that would be caused by higher temperatures. Since the solution employed in the above method contains dispersed ultrafine metal particles, a conductive layer can be formed on plastic film or the like by heating process conducted at a relatively low temperature and under atmospheric pressure. Accordingly, conventional vacuum equipments and complicated processes are not necessary for forming a conductive layer on the substrate.
The above and other objects, features, and advantages of the present invention will be apparent from the following description when taken in conjunction with the accompanying drawings, which illustrates preferred embodiments of the present invention by way of examples.